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The Single UNIX &reg; Specification, Version 2<br>
Copyright &copy; 1997 The Open Group

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<h4><a name = "tag_000_008_357">&nbsp;</a>NAME</h4><blockquote>
semctl - semaphore control operations
</blockquote><h4><a name = "tag_000_008_358">&nbsp;</a>SYNOPSIS</h4><blockquote>
<pre><code>

#include&nbsp;&lt;sys/sem.h&gt;

int semctl(int <i>semid</i>, int <i>semnum</i>, int <i>cmd</i>, ...);
</code>
</pre>
</blockquote><h4><a name = "tag_000_008_359">&nbsp;</a>DESCRIPTION</h4><blockquote>
The
<i>semctl()</i>
function
provides a variety of semaphore control operations as specified by
<i>cmd</i>.
The fourth argument is optional and depends upon the operation requested.
If required, it is of type
<b>union semun</b>,
which the application program must explicitly declare:
<p>
<pre>
<code>
union semun {
            int val;
            struct semid_ds *buf;
            unsigned short  *array;
} arg;
</code>
</pre>
<p>
The following semaphore control operations as specified by
<i>cmd</i>
are executed with respect to the semaphore specified by
<i>semid</i>
and
<i>semnum</i>.
The level of permission required for each operation
is shown with each command,
see
<a href="ipc.html">IPC</a>.
The symbolic names for the values of
<i>cmd</i>
are defined by the
<i><a href="syssem.h.html">&lt;sys/sem.h&gt;</a></i>
header:
<dl compact>

<dt>GETVAL<dd>
Return the value of
<i>semval</i>,
see
<i><a href="syssem.h.html">&lt;sys/sem.h&gt;</a></i>.
Requires read permission.

<dt>SETVAL<dd>
Set the value of
<i>semval</i>
to
<i>arg.val</i>,
where
<i>arg</i>
is the value of the fourth argument to
<i>semctl()</i>.
When this command is successfully executed, the
<i>semadj</i>
value corresponding to the specified semaphore in all processes is cleared.
Requires alter permission, see
<a href="ipc.html">IPC</a>.

<dt>GETPID<dd>
Return the value of
<i>sempid</i>.
Requires read permission.

<dt>GETNCNT<dd>
Return the value of
<i>semncnt</i>.
Requires read permission.

<dt>GETZCNT<dd>
Return the value of
<i>semzcnt</i>.
Requires read permission.

</dl>
<p>
The following values of
<i>cmd</i>
operate on each
<i>semval</i>
in the set of semaphores:
<dl compact>

<dt>GETALL<dd>
Return the value of
<i>semval</i>
for each semaphore in the semaphore set
and place into the array pointed to by
<i>arg.array</i>,
where
<i>arg</i>
is the fourth argument to
<i>semctl()</i>.
Requires read permission.

<dt>SETALL<dd>
Set the value of
<i>semval</i>
for each semaphore in the semaphore set
according to the array pointed to by
<i>arg.array</i>,
where
<i>arg</i>
is the fourth argument to
<i>semctl()</i>.
When this command is successfully executed, the
<i>semadj</i>
values corresponding to each specified semaphore in all
processes are cleared.
Requires alter permission.

</dl>
<p>
The following values of
<i>cmd</i>
are also available:
<dl compact>

<dt>IPC_STAT<dd>
Place the current value of each member of the
<b>semid_ds</b>
data structure associated with
<i>semid</i>
into the structure pointed to by
<i>arg.buf</i>,
where
<i>arg</i>
is the fourth argument to
<i>semctl()</i>.
The contents of this structure are defined in
<i><a href="syssem.h.html">&lt;sys/sem.h&gt;</a></i>.
Requires read permission.


<dt>IPC_SET<dd>
Set the value of the following members of the
<b>semid_ds</b>
data structure associated with
<i>semid</i>
to the corresponding value found in the structure pointed to by
<i>arg.buf</i>,
where
<i>arg</i>
is the fourth argument to
<i>semctl()</i>:
<pre>
<code>
sem_perm.uid
sem_perm.gid
sem_perm.mode
</code>
</pre>
The mode bits specified in
<a href="ipc.html">IPC</a>
are copied into the corresponding bits of the
<b>sem_perm.mode</b>
associated with
<i>semid</i>.
The stored values of any other bits are unspecified.

This command
can only be executed by a process that
has an effective user ID equal to either that of
a process with appropriate privileges or to the value of
<b>sem_perm.cuid</b>
or
<b>sem_perm.uid</b>
in the
<b>semid_ds</b>
data structure associated with
<i>semid</i>.

<dt>IPC_RMID<dd>
Remove the semaphore-identifier specified by
<i>semid</i>
from the system and destroy the set of
semaphores and
<b>semid_ds</b>
data structure associated with it.
This command
can only be executed by a process that
has an effective user ID equal to either
that of a process with appropriate privileges or to the value of
<b>sem_perm.cuid</b>
or
<b>sem_perm.uid</b>
in the
<b>semid_ds</b>
data structure associated with
<i>semid</i>.

</dl>
</blockquote><h4><a name = "tag_000_008_360">&nbsp;</a>RETURN VALUE</h4><blockquote>
If successful, the value returned by
<i>semctl()</i>
depends on
<i>cmd</i>
as follows:
<dl compact>

<dt>GETVAL<dd>
The value of
<i>semval</i>.

<dt>GETPID<dd>
The value of
<i>sempid</i>.

<dt>GETNCNT<dd>
The value of
<i>semncnt</i>.

<dt>GETZCNT<dd>
The value of
<i>semzcnt</i>.

<dt>All others<dd>0.

</dl>
<p>
Otherwise,
<i>semctl()</i>
returns -1 and
<i>errno</i>
indicates the error.
</blockquote><h4><a name = "tag_000_008_361">&nbsp;</a>ERRORS</h4><blockquote>
The
<i>semctl()</i>
function will fail if:
<dl compact>

<dt>[EACCES]<dd>
Operation permission is denied to the calling process, see
<a href="ipc.html">IPC</a>.


<dt>[EINVAL]<dd>
The value of
<i>semid</i>
is not a valid semaphore identifier, or the value of
<i>semnum</i>
is less than 0 or greater than or equal to
<i>sem_nsems</i>,
or the value of
<i>cmd</i>
is not a valid command.


<dt>[EPERM]<dd>
The argument
<i>cmd</i>
is equal to IPC_RMID or IPC_SET
and the effective user ID of the calling process
is not equal to that of a process with appropriate privileges and
it is not equal to the value of
<b>sem_perm.cuid</b>
or
<b>sem_perm.uid</b>
in the data structure associated with
<i>semid</i>.

<dt>[ERANGE]<dd>
The argument
<i>cmd</i>
is equal to SETVAL or SETALL and the value to which
<i>semval</i>
is to be set is greater than the system-imposed maximum.

</dl>
</blockquote><h4><a name = "tag_000_008_362">&nbsp;</a>EXAMPLES</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_008_363">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The fourth parameter in the SYNOPSIS section is now specified as
<i>...</i>
in order to avoid a clash with the ISO&nbsp;C standard when referring to the union
<i>semun</i>
(as defined in XPG3) and for backward compatibility.
<p>
The POSIX Realtime Extension defines alternative interfaces for interprocess communication.
Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
<a href="ipc.html">IPC</a>
can be easily modified to use the alternative interfaces.
</blockquote><h4><a name = "tag_000_008_364">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
None.
</blockquote><h4><a name = "tag_000_008_365">&nbsp;</a>SEE ALSO</h4><blockquote>
<i><a href="semget.html">semget()</a></i>,
<i><a href="semop.html">semop()</a></i>,
<i><a href="sem_close.html">sem_close()</a></i>,
<i><a href="sem_destroy.html">sem_destroy()</a></i>,
<i><a href="sem_getvalue.html">sem_getvalue()</a></i>,
<i><a href="sem_init.html">sem_init()</a></i>,
<i><a href="sem_open.html">sem_open()</a></i>,
<i><a href="sem_post.html">sem_post()</a></i>,
<i><a href="sem_unlink.html">sem_unlink()</a></i>,
<i><a href="sem_wait.html">sem_wait()</a></i>,
<i><a href="syssem.h.html">&lt;sys/sem.h&gt;</a></i>,
<a href="ipc.html">IPC</a>.
</blockquote><h4>DERIVATION</h4><blockquote>
Derived from Issue 2 of the SVID.
</blockquote><hr size=2 noshade>
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UNIX &reg; is a registered Trademark of The Open Group.<br>
Copyright &copy; 1997 The Open Group
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